Diver guarding North Dakota’s capital (6)
I believe the answer is:
london
'capital' is the definition.
(capital of the United Kingdom)
'diver guarding north dakota's' is the wordplay.
'diver' becomes 'loon' (synonyms).
'guarding' is an insertion indicator (inserted letters are protected or guarded).
'north dakota' becomes 'ND' (abbreviation for the US state).
'loon' placed around 'nd' is 'LONDON'.
(Other definitions for london that I've seen before include "English capital" , "City for the non-old, oddly" , "Where Boris Johnson might be" , "Construction across the Thames near the Monument" , "City of Piccadilly" .)